1. Field of the Invention
The current invention relates to the field of position determination. More particularly, the invention relates to position determination using information received from multiple sources.
2. Description of the Related Art
Wireless position determination systems are used to determine the location of a device. Often, the device is a mobile or portable device that may operate from battery power, and the device may not be tethered to any stationary location by a wired communications link.
There are a number of design concerns in a wireless position determination system. Position location accuracy is, of course, one of the concerns. System sensitivity, acquisition time, and power dissipation are also design concerns that are addressed in a position determination system. Wireless position determination systems typically incorporate a trade-off of design constraints in an attempt to obtain a relative optimization of each of the system concerns.
As wireless communication systems become more popular, the desire to incorporate some type of position location capability has emerged. In a wireless communication system, such as a wireless telephone system, it may be desirable to be able to locate the position of a mobile device such as a wireless telephone handset. Indeed; in the United States, enhanced emergency wireless service having the capability of determining the location of a handset has been mandated for wireless phone providers. Wireless service providers, in conjunction with equipment manufacturers, have devised a variety of position location systems that are able to provide the location of a mobile device, such as a portable handset. Each of these location systems emphasizes different system concerns and works by differing mechanisms.
One position location system that may be utilized by a mobile device is the Global Positioning System (GPS). In the Global Positioning System, there are approximately twenty-four satellites that orbit the earth. Each of the satellites transmits a carrier frequency that is modulated with a pseudo random noise (PRN) code sequence. The PRN code that is commonly used by civilian based GPS receivers is termed the Coarse Acquisition (C/A) code. Each satellite transmits a different PRN code. In the GPS, a GPS receiver receives the signals from multiple satellites and determines the distance from each satellite in order to trilaterate the position of the receiving device.
As an example, a receiver that is approximately synchronized to GPS time receives a signal from a first GPS satellite and demodulates the received carrier frequency to obtain the PRN code. The receiver determines a pseudo range, or uncorrected distance measurement, to the first satellite by correlating an internally generated PRN code to the received PRN code. The pseudo range thus defines a surface of a sphere centered at the satellite. The receiver determines its location by determining pseudo ranges to other satellites and calculating the intersection of the corresponding sphere surfaces.
Although the accuracy of GPS position location is excellent for purposes of locating a handset, the time to acquire a first position fix may be long, varying up to several minutes. Additionally, the ability to receive signals from a plurality of satellites is impeded in environments where the signals from satellites may be occluded by the presence of tall structures or overhead foliage. As is known, a GPS receiver typically must receive signals from at least four satellites in order to determine its position accurately.
Another position location system that may be used by wireless phones is based on Cell-ID. Wireless phones register with the wireless system such that the wireless system knows with which base station the wireless phone is communicating with. Additionally, some base stations may be sectorized and the wireless system is able to identify a particular sector of the base station the phone is communicating with. The position of a wireless phone may then be determined according to the cell or sector in which the wireless phone is registered or communicating with. This type of position location service may be inherent within many types of wireless systems, but unfortunately only provides a very coarse position location that varies according to the size of the cell.
Another position location system that may be used by wireless devices is Enhanced Observed Time Difference (E-OTD). E-OTD is a position location system that is optimized for use in Global System for Mobile communications (GSM) and General Packet Radio Service (GPRS) wireless communication systems. In this system, the mobile device monitors transmission bursts from multiple base stations and measures the time shifts between the arrival of frames in order to determine its position. The mobile device must receive signals from at least three base stations in order to make a position determination. However, the E-OTD system requires the use of Location Measurement Units (LMUs) strategically placed throughout the network in order to provide the system with the precise timing required to make the position location relatively accurate. Additionally, position determination may not be possible in some service areas because the mobile device cannot communicate with at least three base stations.
Another position location system that may be used by wireless phones is Observed Time Difference of Arrival (OTDOA). OTDOA is a position location system that is optimized for use in Wideband Code Division Multiple Access (WCDMA) systems. The OTDOA position location system operates similar to the E-OTD system. The location of a mobile device is estimated by determining the time difference of arrival of communication signals from multiple base stations. In addition to requiring timing units similar to the LMUs required in the E-OTD system, the problems associated with not communicating with a sufficient number of base stations are further aggravated by the use of WCDMA, which utilizes power control. Power control minimizes the transmit power required to achieve a desired quality of service. Because transmit power from the base station is minimized, the probability that the mobile device is communicating with the necessary three base stations is reduced.
Still another position location system that may be used by wireless phones is wireless Assisted GPS (A-GPS). In A-GPS, signals from GPS satellites, as well as signals received from base stations in the wireless system, are used for position location. An A-GPS may be configured to operate in a system where the mobile device acquires satellite (and other timing information), calculates pseudo ranges corresponding to the timing information, and sends the pseudo range information to an A-GPS location server where the actual position of the mobile device is determined. In an alternative configuration, the mobile device performs the position determination itself without using the location server. In both alternatives, a location server may be used to provide aiding data to the mobile device to assist in the acquisition of satellite signals. The aiding data greatly reduces the time required to compute a first fix because the search performed by the mobile device may be bounded by the aiding data and the need to receive and demodulate satellite navigation data is eliminated.
Still other position location systems may use a combination of position location systems. Hybrid position location systems typically incorporate signals from at least two different position location sub-systems in order to determine the location of a mobile device. The received signals may be used mutually exclusively or may be used in combination when making the position determination.
A-GPS may be viewed as a hybrid position location system using information from both a location server as well as GPS satellite information. Still other position location systems may use GPS satellite signals in addition to timing and pseudo range information derived from wireless base station signals. For example, U.S. Pat. No. 5,999,124, by Sheynblat, entitled “SATELLITE POSITIONING SYSTEM AUGMENTATION WITH WIRELESS COMMUNICATION SIGNALS” discloses a system in which the position of a receiver is determined from a combination of position information from a satellite system and from a wireless communication system. In other position location systems, time of arrival information from a first position location sub-system may be used in conjunction with cell ID information from a wireless communication system used as a second position location sub-system.
However, aside from GPS, each of the above mentioned position determination systems requires a mobile device to be in communication with a fixed position determination sub-system. Many of the position determination systems require the mobile device to be part of a wireless phone system. Not all mobile devices are part of wireless phone systems. For example, wireless communication systems may comprise a number of two-way radios, or other independent mobile devices. The mobile devices in some systems may communicate directly with each other as well as to a fixed base station. It would be advantageous to allow each of the mobile devices in such a wireless communication system to determine its position. However, as noted earlier, use of GPS alone may not provide a satisfactory solution. There may be a low probability of receiving a sufficient number of satellite signals, especially in an urban environment where buildings and other man-made or natural structures often occlude the satellite signals. For these reasons, what is needed is a system that provides a wireless device with accurate position information, but does not require the device to directly communicate with the minimum number of satellites to independently determine its position.